1. Field of the Invention
The present invention relates to an actuator for electromagnetic valve control with two electromagnets, between which a retaining plate with at least one tappet oscillates, and which in each case consists of a yoke with a coil window and an exciting coil.
2. Description of the Related Art
An actuator for electromagnetic valve control substantially consists of the opening magnet and the closing magnet, which are separated from one another by at least one component made of a nonferromagnetic material. This component can, for example, be a housing part. The retaining plate made of a ferromagnetic material is located between the electromagnets, the opening magnet and the closing magnet; it is moved in the appropriate direction by applying current to the exciting coil of the opening magnet or the exciting coil of the closing magnet. The yoke of the opening magnet has a bushing for a valve tappet which transmits forces acting upon the retaining plate to at least one intake/exhaust valve.
The actuator can, for example, be so designed that the actuator spring is arranged on the outside of the closing magnet on the side of the actuator opposite to the intakelexhaust valve. For this, a spring tappet having an actuator spring plate is arranged as an extension of the valve tappet, the spring tappet being seated by a bushing in the yoke of the closing magnet. The yoke of the closing magnet has a formed shape, in which an internal thread is tapped and which creates a wall around the bushing of the spring tappet. A screw cap is screwed into the internal thread of the wall which screw cap, together with the wall, forms a hollow space in which the actuator spring is arranged lying on the actuator spring plate. The pretensioning of the actuator spring can be changed by turning the screw cap, thus making the rest position of the retaining plate adjustable.
An actuator together with an intake/exhaust valve forms a functional unit in which the intake/exhaust valve, corresponding to a conventional cylinder head with camshafts, is pulled into the valve seat of the cylinder head by a valve spring and a valve spring plate.
If a functional unit consisting of an actuator and an intake/exhaust valve is mounted on the internal combustion engine, then the valve stem of the intake/exhaust valve, the valve tappet and the spring tappet of the actuator are pressed against each other. In the rest position of the functional unit, the retaining plate is located in the middle between the opening magnet and the closing magnet, with the valve spring and the actuator spring being pretensioned. Here, the valve plate of the intake/exhaust valve is located in the middle position between the valve seat of the cylinder head, in which the intake/exhaust valve is closed, and the position in which the intakelexhaust valve is maximally opened.
As a result of the continually changing magnetic fields of the electromagnets during the operation of an actuator for electromagnetic valve control, eddy currents arise in the yokes of the electromagnets which, in accordance with Lenz's Law, counteract the build-up and degradation of the magnetic field. During operation of the actuators this results in an increase in power consumption and an increase in undesirable heating of the actuators.
In order to prevent the formation of eddy currents, U.S. Pat. No. 4,715,331 proposes, for example, that the yokes of the electromagnets be composed of low-retentive metal sheets analogous to the iron core of a transformer.
The disadvantages of such laminated yokes lie in the low mechanical strength of the laminated core and in the high number of individual parts which have to be handled and machined during the manufacture of the electromagnets.